digraph G {
  a[label="", shape=rectangle];
  b[label="", shape=rectangle];
  c[label="", shape=rectangle];
  {rank=same; b,c}
  d[label="[4]", shape=circle];
  e[label="[2]", shape=circle];
  f[label="[6]", shape=circle];
  g[label="[1]", shape=circle];
  {rank=same; d,e,f,g};
  a -> b;
  a -> c;
  b -> d;
  b -> e;
  c -> f;
  c -> g;
}
